(a) Field of the Invention
This invention relates to an antioxidant barrier for carbon-based material and also to a process of making this antioxidant barrier. More particularly, the invention relates to the lateral protection of graphite electrodes of electric arc furnaces by providing antioxidant coatings on said electrodes.
(b) Description of Prior Art
It is known that carbon-based materials, such as graphite possess good mechanical properties at high temperature and are resistant to thermal shock. This is the reason why graphite has been extensively used as the material of the electrode in arc furnaces. However, above 500.degree. C., graphite oxidizes very rapidly unless it is coated with an antioxidant barrier. On the other hand, the lateral loss of graphite electrodes which are used in electrical arc furnace for melting steel is the result of oxidation. The prior art has reported that 50% of the total graphite consumption in electric arc furnace could be attributed to lateral loss.
Obviously, to reduce this highly pronounced oxidation which takes place at high temperature, the graphite must be protected against any hostile medium by means of a coating or the like. It has also been found that any coating intended to protect graphite against oxidation should satisfy certain requirements. For example, the coating selected should prevent or at least inhibit the diffusion of oxygen and carbon. This coating should be also mechanically compatible with graphite at any temperature and resistant to thermal shock.
Numerous attempts to protect arc furnace electrodes by means of a protective coating have failed to succeed. For example, the coatings of German Patent 1,009,093 which are silver based are too expensive to become commmercial. On the other hand, by utilizing the teaching of British Patent No. 1,026,055 and Canadian Patent No. 986,376, graphite is not really protected against attack by oxidation and against thermal shock.
However, Canadian Patent No. 706,351 and U.S. Pat. No. 3,553,010, both disclose a composite coating which presumably protects the graphite electrode against oxidation. The Canadian Patent No. 706,351 relates to a coating which comprises two layers:
(a) 1st layer: a refractory carbide (TiC, CrC, MoC, SiC, ZrC):
(b) 2nd layer: a refractory alloy or compound of an element of groups IVa to VA of the periodic system with nitrogen, beryllium, boron, aluminum, silicon and phosphorus.
According to the process of Canadian Patent No. 706,351, the element, for example a graphite electrode, must be coated with titanium carbide or titanium carbide is formed thereon, after which the second coat is applied. It has been found that the formation of a coat of titanium carbide is detrimental to the application of the second coat, which is the oxidant protection coat, the coat of titanium carbide reacts with aluminum and silicon to form compounds of poor mechanical properties and coating spalls off easily.
U.S. Pat. No. 3,553,010 describes a first layer of silicon with or without sodium, magnesium, calcium, boron, aluminum, titanium, zirconium, manganeses, iron, carbon, nitrogen, phosphorus and oxygen separately or in combination. The second layer called the cover layer comprises aluminum with or without sodium, magnesium, boron, silicon, phosphorus, oxygen, copper, zinc, lead, titanium, ziconium, chromium, manganese, iron, cobalt, and nickel, separately or in combination. This composite coating is not completely satisfactory against corrosion.
Tests have been made with various coatings in order to determine their suitability to provide a good antioxidant barrier for a graphite electrode. For example, coatings of graphite electrodes have been produced from oxides, oxide cermets, borides, carbides, metals silicides or composites thereof, but none of these have appeared satisfactory. For examples coatings based on oxides and those made of oxide cermets have a definite tendency to crack which leads to the coatings being slowly detached from the graphite electrodes. On the other hand coatings derived from carbides and borides do not afford a sufficient protection against oxidation since losses of up to 30% of the electrode have been found. Metal coatings offer a good protection against oxidation for a limited time, however they are too easily detached from the graphite electrode to be seriously considered as an antioxidant barrier for graphite electrodes. Finally, silicides and their composites have low resistance to thermal shocks and crack easily.
It is therefore an object of the present invention to provide a coating for carbon-based materials which constitutes an antioxidant barrier.
It is another object of the present invention to provide a process for producing coatings for carbon-based materials which behave as antioxidant barrier.
It is another object of the present invention to ensure the lateral protection of graphite electrodes (e.g. steel-furnace electrodes) by reducing consumption of carbon by combustion in oxygen (burn-off).
It is another object of the present invention to provide a coat which becomes more adherent to the carbon-based material and eventually leads to a stable mixture which is relatively unaffected by oxidizing conditions.
It is another object of the present invention to provide an antioxidant barrier for graphite electrode, based on the use of metallic titanium or alloys thereof, silicon carbide and aluminum or an alloy thereof.